Developmental failure of segmentation in a caudal vertebra of Apatosaurus (Sauropoda).

A vertebral element assigned to an Apatosaurus cf. ajax from the Late Jurassic Morrison Formation is described. The specimen exhibits an unusual morphology where two vertebrae are nearly seamlessly fused together, including the haemal arch that spans them. This morphology is thought be the result of a developmental abnormality. CT scans of the specimen reveal a thin zone of dorsoventral thickening between the two neural arches consistent with cortical bone. Contrast in internal morphology differentiates the anterior and posterior vertebral bodies with the anterior expressing greater porosity, which increased accommodation for barite-rich calcite precipitation. No vacuities are observed to suggest the former presence of an intervertebral disk or intervertebral joints: the absence of an intervertebral disc or intervertebral joints is indicative of a condition known as block vertebra. Block vertebrae occur with the loss, or inhibition, of somitocoele mesenchyme early in embyogenesis (i.e., during resegmentation of the somites responsible for the formation of the affected vertebra). The derivatives of somitocoele mesenchyme include the intervertebral disc and joints. Although vertebral paleopathologies are not uncommon in the fossil record, this specimen is the first recognized congenital malformation within Sauropoda. Anat Rec, 297:1262-1269, 2014.